To meet power demands varying depending on time periods, peak shaving or peak shift has been implemented to charge a storage battery in a time period with lower power demands and to discharge the storage battery in a time period with higher power demands. A storage battery undergoes cycle degradation and storage degradation. Cycle degradation refers to a phenomenon in which a battery capacity decreases due to repeated charge and discharge. Storage degradation refers to a phenomenon in which a battery capacity decreases with a lapse of time regardless of whether a battery is used. Both the cycle degradation and the storage degradation are caused by an irreversible chemical reaction in the storage battery.
In a storage battery of which the state of charge (SOC) is higher, storage degradation progresses increasingly. In a storage battery of which the temperature is higher, storage degradation also progresses increasingly since the high temperature facilitates a chemical reaction in the storage battery. Hence, it has been considered to lower a charge-target SOC of a storage battery of which the temperature is high (see Patent Literature 1, for example).